Fixing substitution of z regions
Fix substitution of RPLs for 'this'

Previously substitution of RPLs for this was broken:

1. It was done with a separate mechanism, instead of as a special case
of substituting for a local variable.

2. It didn't work right, because different symbols representing 'this'
in different contexts were not differentiated.

Those deficiencies have been corrected, by making substitution for
'this' work the same way as substitution for any other local variable.